One known type of information storage device is a disk drive device that uses magnetic media to store data and a movable read/write head that is positioned over the media to selectively read from or write to the disk.
Consumers are constantly desiring greater storage capacity for such disk drive devices, as well as faster and more accurate reading and writing operations. Thus, disk drive manufacturers have continued to develop higher capacity disk drives by, for example, increasing the density of the information tracks on the disks by using a narrower track width and/or a narrower track pitch. However, each increase in track density requires that the disk drive device have a corresponding increase in the positional control of the read/write head in order to enable quick and accurate reading and writing operations using the higher density disks. As track density increases, it becomes more and more difficult using known technology to quickly and accurately position the read/write head over the desired information tracks on the storage media. Thus, disk drive manufacturers are constantly seeking ways to improve the positional control of the read/write head in order to take advantage of the continual increases in track density.
One approach that has been effectively used by disk drive manufacturers to improve the positional control of read/write heads for higher density disks is to employ a secondary actuator, known as a micro-actuator, which works in conjunction with a primary actuator to enable quick and accurate positional control for the read/write head. Disk drives that incorporate micro-actuators are known as dual-stage actuator systems.
Various dual-stage actuator systems have been developed in the past for the purpose of increasing the access speed and fine tuning the position of the read/write head over the desired tracks on high density storage media. Such dual-stage actuator systems typically include a primary voice-coil motor (VCM) actuator and a secondary micro-actuator, such as a PZT element micro-actuator. The VCM actuator is controlled by a servo control system that rotates the actuator arm that supports the read/write head to position the read/write head over the desired information track on the storage media. The PZT element micro-actuator is used in conjunction with the VCM actuator for the purpose of increasing the positioning access speed and fine tuning the exact position of the read/write head over the desired track. Thus, the VCM actuator makes larger adjustments to the position of the read/write head, while the PZT element micro-actuator makes smaller adjustments that fine tune the position of the read/write head relative to the storage media. In conjunction, the VCM actuator and the PZT element micro-actuator enable information to be efficiently and accurately written to and read from high density storage media.
One known type of micro-actuator incorporates PZT elements for causing fine positional adjustments of the read/write head. Such PZT micro-actuators include associated electronics that are operable to excite the PZT elements on the micro-actuator to selectively cause expansion and/or contraction thereof. The PZT micro-actuator is configured such that expansion and/or contraction of the PZT elements causes movement of the micro-actuator which, in turn, causes movement of the read/write head. This movement is used to make faster and finer adjustments to the position of the read/write head, as compared to a disk drive unit that uses only a VCM actuator. Exemplary PZT micro-actuators are disclosed in, for example, JP 2002-133803; U.S. Pat. Nos. 6,671,131 and 6,700,749; and U.S. Publication No. 2003/0168935, the contents of each of which are incorporated herein by reference.
FIG. 1a illustrates a conventional disk drive unit and show a magnetic disk 101 mounted on a spindle motor 102 for spinning the disk 101. A voice coil motor arm 104 carries a head gimbal assembly (HGA) that includes a micro-actuator 105 with a slider incorporating a read/write head 103. A voice-coil motor (VCM) is provided for controlling the motion of the motor arm 104 and, in turn, controlling the slider to move from track to track across the surface of the disk 101, thereby enabling the read/write head 103 to read data from or write data to the disk 101.
Because of the inherent tolerances (e.g., dynamic play) of the VCM and the head suspension assembly, the slider cannot achieve quick and fine position control, which adversely impacts the ability of the read/write head to accurately read data from and write data to the disk when only a servo motor system is used. As a result, a PZT micro-actuator 105, as described above, is provided in order to improve the positional control of the slider and the read/write head 103. More particularly, the PZT micro-actuator 105 corrects the displacement of the slider on a much smaller scale, as compared to the VCM, in order to compensate for the resonance tolerance of the VCM and/or head suspension assembly. The micro-actuator enables, for example, the use of a smaller recording track pitch, and can increase the “tracks-per-inch” (TPI) value for the disk drive unit, as well as provide an advantageous reduction in the head seeking and settling time. Thus, the PZT micro-actuator 105 enables the disk drive device to have a significant increase in the surface recording density of the information storage disks used therein.
These refinements have focused on finely tuned horizontal displacement to accommodate the rapid increase in disk drive capacity. Similarly, rapidly increasing the capacity also requires that the height at which the head flies over the magnetic media be controlled with more and more sensitivity. Accordingly, an acceleration sensor and/or pressure sensor has been provided between the suspension dimple and the flexure of an HGA as disclosed, for example, in JP 2005-093055, the entire contents of which are incorporated herein by reference. When the head flying height changes, the acceleration sensor and/or the PZT sensor will detect the pressure between the dimple and the flexure and generate an electrical potential voltage in response thereto. From this signal, the servo will adjust and/or compensate for the changes in flying height.
FIG. 1b is a sensor for detecting flying height in the prior art. An acceleration sensor or pressure sensor 115 is a laminated structure, located between the dimple 112 formed on the load beam 111 and the slider 100. The sensor 115 includes a piezoelectric crystal layer 119. The first and second conductor layers 118 and 120 are formed on both sides of the piezoelectric crystal layer 119. A first insulator layer 117 is disposed between the first conductor layer 118 and the metal layer 116 (which may contact dimple 112). A second insulator layer 121 may be disposed between the second conductor layer 120 and the slider 100. When head-disk interface (HDI) occurs, the acceleration sensor or pressure sensor 115 will be pressured, generating an electrical potential voltage of several millivolts. Based on this signal, the servo will adjust and/or compensate the flying height.
Unfortunately, this technique suffers several drawbacks. For example, because of size constraints, the sensitivity to flying height changes is limited. Also, the sensitivity frequently changes when an environmental condition changes. Thus, for example, as the altitude changes, the sensitivity of the altitude measurement also changes, which challenges the servo control system to account both for the change in height and the change in height measurement sensitivity. Moreover, prior techniques provide a PZT element between the suspension flexure and the dimple, which may make it easy to damage the PZT element during dimple and flexure interference (e.g. when a shock or vibration occurs, etc.). This interference may generate fragments or particles, which may, in turn, contaminate the head-disk interface and affect the read and write functions of the head. In the long-term, these drawbacks result in reliability concerns. Additionally, the manufacturing process is difficult and costly.
Thus it will be appreciated that there is a need in the art for altitude sensing systems for flying height adjustment in disk drive devices.